howardc64

I have a front derailleur that is held in place by the bottom bracket against the frame. I would tighten the bottom bracket tight against the derailleur mount and against the frame. Everything is nice and tight. But after I put on the crank, the derailleur would no longer be held tightly and have a little looseness. It is just a little bit loose and doesn't get worse.

What am I doing wrong?

DCB0

You have what is called an E-type front derailleur. I do not know what happened to A- through D type front derailleurs.

THere are three things I can think of:
1. E-type derailleurs should be used with bottom brackets designed for E type derailleurs. I am not sure what the difference is (might just be spacing compensation for the extra witch the E-type mount adds to the BB shell), but if you have the wrong type, perhaps it is not held in place well.
2. The derailleur is, IIRC, held to the E-type plate with a screw and can be adjusted to move the derailleur up and down. Is it possible that this screw is not tight and allowing some movement.
3. It may be that nothing is wrong and there is a little bit of wiggle in the derailleur from wear or poor QC from the factory... and perhaps you just don't notice the wiggle when there is no crank installed because there is no fixed object next to the derailleur to which you can compare it... like how a rim that is reasonably straight and true on a bike can look more wobbly when put in a truing stand and compared to the fixed feelers.

howardc64

Thanks DCB0, found my problem

BB and the E Type mount is mismatched.

Tighten the BB all the way leaves 3mm gap
E type mount thickness is 2.6mm

Thus, I have play. Here is a pic of the gap.



BB model is Shimano BB-LP27-E. I'm guessing -E means E type mount. I wonder why it doesn't fit?

Front Derailleur is Shimano Acera.

jimc101

How is this moving? you need to bolt the FD into the Bottom Bracket Mount, if you are using a frame designed for an E-Type, then this is a mount above the BB shell, if this isn't present, you are using the wrong type of FD for the frame (your photos don't show this)

The fitting instructions are here https://techdocs.shimano.com/media/te...9830648142.pdf for an E-Type BB, thinking as your frame doesn't have the mount, you need to replace the BB with the correct size, from the info given, you have fitted a 73mm shell BB, when you need a 68mm shell BB, as your current setup will have a detrimental effect on the chainline. Trying to use a large washer to fix this isn't the going to fix the chainline.

howardc64

Thanks Jim. I should have posted more pics. Here they are

https://picasaweb.google.com/howardc64/BottomBracket#

BB is Shimano BB-LP27-E 73mm
FD is Acera
Width of the frame where the BB goes is 73mm

The FD plate that clamps between the BB and the frame is loose because the plate's thickness is less than the gap between BB and the frame.

The FD also doesn't shift the full range covering the 3 chain rings.

Got the bike used so just trying to figure out which part is mismatched. Bike is Trek 4900, didn't see it in Trek's archive going back to 2003 so probably a little over 10 years old.

Airburst

I'm fairly sure the E-type Acera front mech I removed from a friend's bike (singlespeed conversion) wasn't held on by anything other than the bottom bracket. I'm also fairly sure the similar-looking one that's on one of my bikes has some sort of clip that goes about halfway around the seat tube, rather than any sort of bolt.

Edit. I'm not sure the derailleur in your link and the one in the OP's picture are the same design. What the OP has is definitely one of the ones that's held on by the bottom bracket, the one in your link seems to just bolt to the frame. And he may not have the wrong bottom bracket, they're meant to stick out about that far because they clamp the front derailleur on. When I removed the one I took off, I had to put a spacer between the BB cup and the frame, where the derailleur plate had been, because the chainline was out of whack without it.

jimc101

For the Acera, looks like you are missing the 2.5mm spacer https://techdocs.shimano.com/media/te...9830616270.pdf

Airburst

It just hit me. The E-type mount has those two threaded lugs on the frame, and that thing on all the E-type front mechs I've ever seen is an adaptor to allow E-type derailleurs to be fitted to a frame without the lugs! My apologies, that IS in fact an E-type derailleur in the link, but with the correct frame fitting.

howardc64

Bingo... looks like 2.5mm spacer is what I need. This also helps get the FD to shift further out towards the big chain ring as it can't quite travel that far right now.

Thanks!!!

EDIT: That was it. Got a spacer from a local bike shop and FD's BB mount is now nice and tight. Now I have to figure out how to get the FD to shift the full range and catch the big gear on the triple crank